This invention relates to an electrooptic probe for test and characterization of ultra-fast circuits, such as the picosecond fast circuits being used in optical communication systems and the method of using the probe.
For the measurement of ultra-high frequency signals in electronic circuits, such as those used in optical communication systems, it is advantageous to have a freely positionable probe whose speed of response is even faster than that of the signals being measured. For this purpose, there have been developed probes that use photoconductive switches. Typical of probes of this kind is the photoconductive sampling probe described in a paper entitled xe2x80x9cA Fiber-Mounted Micromachined Photoconductive Probe with 15 nV/Hzxc2xd Sensitivityxe2x80x9d, App. Phys. Letter 69 Sep. 13, 1996.
This probe comprises as the switching region a film that has been grown on a standard monocrystalline gallium arsenide substrate by low temperature molecular beam epitaxy and that is generally described as LT GaAs. In an article in Applied Physics Letters, Vol. 59, Sep. 16, 1991, pp. 1491-1493, entitled xe2x80x9cLow-Temperature Crown (GaAs Quantum Wells: Femtosecond Nonlinear Optical and Parallel-Feed Transport Studies,xe2x80x9d LT GaAs material is shown to include semi-insulating electrical characteristics, subpicosecond photoconductive decay time and reasonably high mobility making possible the generation of subpicosecond electrical pulses. This high impedance probe has microvolt sensitivity with picosecond time. resolution and megohm impedance, thus enabling non-invasive tests of ultra-high speed circuits. However, these probes needed to be designed to be very thin to allow the short light pulses that are used to activate the probe to be transmitted sufficiently through the thickness of the absorptive standard GaAs substrate of the probe to make conducting the photoconductive LT GaAs switching film. Typically with such probes, the LT GaAs is grown on a substrate that was initially at least hundreds of microns thick. Subsequently the substrate typically has been thinned to no more than one or two microns and preferably is removed essentially entirely. As a result such probes tend to be difficult and expensive to make and also are quite fragile, limiting their use. Moreover, generally the circuitry of the device under test is at its top surface making it impractical to transmit the activation light pulses through the device under test to the probe.
The desirable properties of LT GaAs mentioned above, particularly the short photoconductive decay time and high mobility, are primarily the result of the presence in such material of excess arsenic that provides dangling bonds or defect sites that capture quickly the free carriers generated by the excitation light. The excess arsenic occurs because LT GaAs generally is grown at temperatures in the range of about 190xc2x0 C. and 300xc2x0 C. in contrast to the range of between about 600xc2x0 C. and 700xc2x0 C. normally used for growing standard GaAs and in an atmosphere of excess arsenic.
The present invention provides an optoelectronic probe that includes on one surface a photoconductive switch, advantageously a relatively thin film of LT GaAs, that is optimized for two-photon operation. As such, it can be activated by an activation light beam of multiple excitation photon energy that irradiates the opposite surface of the probe. Such a beam can pass through with little absorption a relatively thick substrate, advantageously at least several hundred microns thick, supporting the switching layer so long as the excitation photon energy of the beam is below the bandgap of the substrate material of the probe. Advantageously, for two-photon excitation the light can be provided by a mode-locked femtosecond pulsed laser of 1.55 microns wavelength of the kind known in the art. As a consequence, there is no need to thin the substrate, since thicknesses of several hundred microns are now tolerable. Moreover, if there is chosen an activation light of appropriate excitation photon energy, alternatively the light beam can be applied to pass first through the device under test to activate the switching film of the probe.
In addition to the use of a film of LT GaAs, a GaAs film having dangling bonds and/or captive sites sufficient to provide the desired short photoconductive decay time and high mobility can be provided in other ways. These include Erbium doping, and ion or proton bombardment.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.